Some dental treatment systems use lasers, e.g., for cutting tooth enamel. It may be beneficial to replace drills conventionally used in dental treatment with laser-based systems, in part because lasers can provide for better control of the process of cutting and removing material from a tissue to be treated, such as a tooth. Also, laser-based systems are typically less noisy and may produce less vibrations than a conventional drill. Lasers have also been found to be useful in the removal of dental material with less amount of local anesthetic than that required when the procedure is performed with a drill. For these reasons many in the dental industry have expected for some time that laser based systems will replace the drill based dental treatment systems.
The Food and Drug Administration (FDA) has approved seven different types of lasers and laser diodes for use in dental applications. Lasers used to treat soft tissue are chosen according to the water absorption characteristics of the laser, and various other characteristics so that the blood is cauterized and bleeding is minimized. Many of the dental treatment systems, however, cut the enamel or hard tissue using lasers that exhibit high water absorption. Typically, the applied laser energy is absorbed in water in tooth enamel (about 4% by volume), causing the water to vaporize, and the resulting steam causing to facture the hard tissue/enamel and thus removing a portion of the enamel from the tooth. Because the laser wavelength is chosen such that a significant portion of the laser energy is strongly absorbed by water, a common problem with such systems is that the cutting operation is slow, due to the low percentage of water in the enamel. In fact, this method of removing dental hard tissue is often considerably slower than using dental drills and many dental procedures are completed in less time by an operator/dentist using a conventional drill than by using a laser that operates by absorption of laser energy in water. For this reason may practitioners choose not to use laser based systems for dental treatment, and as such the benefits that laser based treatment can offer are not widely available at present.
Laser wavelengths in the range of 5 to 15 μm are strongly absorbed by the hydroxyapatite that makes up to about 96% of tooth enamel by weight. Therefore, using lasers in the range of about 5 μm up to about 15 μm can result in faster cutting of hard tissue than that using known laser based dental treatment systems. As explained below, cooling is particularly important in using lasers in the 5 to 15 μm range.
The use of a fluid to assist in cleaning during cutting, e.g., for removing particles created during the cutting operation is known. Such fluids may also be used for cleaning prior to and following the cutting operation. In general in laser-based dentistry, without adequate cooling enamel melts when ablated by a laser of any wavelength, forming non-apatite CaP phases. Therefore, when a laser is used for cutting, a fluid can be used to cool the tissue within the treatment area and/or the surrounding area, to prevent thermal damage thereto, in addition to cleaning.
When the laser is used to fracture the tooth structure by water vaporization, water provided by the system to cool the treatment area can interfere with the laser beam without a significant reduction in material removal rates. This is because the coolant, that may absorb the laser radiation and vaporize can serve to remove hard tissue material by imparting disruptive forces to the treatment area. In some systems, the coolant system is intentionally configured such that the coolant interferes with the laser beam to impart disruptive forces on dental hard tissue. In some laser based dental treatment systems, the coolant is provided only to avoid or minimize the melting of the enamel, without regard to whether the coolant interferes with the ablation operation of the laser beam. Some systems that deliver coolant between laser pulses do not provide forced convective cooling from the surface of the dental region being treated at or nearly at the time the surface is being heated. This can cause the treated surface to experience thermal cycles, heating during laser pulses and cooling in between such pulses.
A dental laser system in which ablation occurs, at least in part, due to absorption of the laser energy in the hydroxyapatite of the hard tissue is likely to experience a marked reduction in material removal rate when a cooling fluid interferes with the laser. In such systems, providing unregulated quantities of water or other coolants is often not a beneficial solution to solve the problem of the melting of the enamel. This is because hydroxyapatite absorbs laser energy at 9.3 to 9.6 μm (which can be classified mid to far infrared range of the laser wavelength) but water also absorbs laser energy at 9.3 to 9.6 μm, and as such, the cooling fluid may significantly attenuate the laser power used for ablation. Therefore, an excess amount of water can interfere with the cutting operation of the laser beam.
In addition, though the melting of the enamel can decrease the efficiency of cutting in general, this problem can be significant when mid to far infrared lasers are used. This is predominantly because the CaP phase structure has a completely different absorption characteristic than hydroxyapatite such that far-infrared 9.3-9.6 μm energy goes from being highly absorbed to virtually not absorbed in the CaP phases. Therefore, this shift in absorption mechanism can make the cooling and ablation of enamel at mid to far-infrared wavelengths far more critical than near-infrared (e.g., conventional) wavelengths where the absorption mechanism is water. Because the ablation of the hydroxyapatite and non-apatite CaP phases is not that dissimilar, some enamel melting that can be tolerated when cutting with near infrared (conventional) laser energy, because the ablation is generally caused by evaporated water. It is highly desirable to avoid such melting when mid to far infrared energy laser is used for ablation because the CaP phases can substantially mitigate or even prevent absorption of the laser energy in the hard tissue to be removed.
As such, there is a need for improved systems and methods to control the fluid used to cool the tissue during laser processing thereof to provide for sufficient removal rates while substantially preventing melting of the enamel.